1. Field of the Invention
The present invention relates generally to broadcast receivers, and more particularly to broadband tuners utilizing a local oscillator.
2. Related Art
Broadcast satellites transmit signals over multiple channels across a broadband spectrum. These satellites may be combined with receiving systems to create a direct broadcast satellite system (DBS). A current example of such a system is Digital Satellite Service/Direct Video Broadcast (DSS/DVB), which broadcasts over the range 12 to 14 GHz. A DSS/DVB receiving system generally includes an antenna, a low-noise block converter (LNB) and a direct down-conversion broadband receiver. The direct down-conversion receiver comprises a tuner, an analog-to-digital converter and a baseband demodulator. The LNB converts the 12–14 GHz band signal received by the antenna into a 950–2150 MHz band signal. The tuner directly converts this 950–2150 MHz band down to baseband, which consists of in-phase (I) and quadrature (Q) components. These I and Q components are then transformed into digital data by the analog-to-digital converter and demodulated by the baseband demodulator to decode the video and audio being broadcast.
The tuners in such systems typically utilize mixers to perform the down-conversion from 950–2150 MHz to I and Q components. These mixers require a local oscillator, which typically consists of a frequency synthesizer with a step size equal to the receive channel spacing. Such local oscillators are often implemented using a frequency synthesizer and a voltage controlled oscillator. The signal from this local oscillator drives the mixers. Commonly, the local oscillator operates at the same frequency as the carrier frequency of the selected channel.
However, some local oscillators are designed to operate at approximately twice the frequency of the carrier signal. This is done to avoid the many problems, such as cross modulation and direct current (DC) offset, which arise when the local oscillator operates at exactly the same frequency as the carrier. Thus in these systems, the local oscillator signal is passed through a divider before being fed to the mixers. Additionally, in some of these designs, the phase shift required to obtain the Q component is also performed within the divider. However, even in these types of systems, a strong RF signal can modulate the local oscillator signal, thus causing tuner degradation.
Typically, the local oscillator consists of two external voltage-control oscillators (VCOs). The external VCOs are typically off-chip tank circuits, residing inside the set-top box and using the common transformer as a power source. These tank circuits commonly include four inductors and two hyper-abrupt varactor diodes. The external hyper-abrupt varactors commonly use 30 volts of tuning range, and have a ten-to-one capacitance, thus providing the large tuning range needed for broadband signals.
The problems with this traditional approach to broadband tuners are multiple. First, when the tank components are off-chip, the inductors will radiate like an antenna. Thus, these traditional tuners create high frequency radiation, which must be dealt with to avoid interference with other components on the board. Second, the common solution to the unwanted radiation is to add shielding. This solution makes the tank components larger and more difficult to integrate onto the board, thus requiring more board space and increasing the cost of producing the tuner. Third, the traditional off-chip tank circuits include 30 V hyper-abrupt varactor diodes, thus requiring the set-top box to include a 30 V tap off the transformer. This also increases the cost of producing the tuner by taking up valuable space inside the set-top box.
Integrated wideband tuners have been reported using multiple wideband RC oscillators. While these tuners reportedly solve the problems of coupling between the radio frequency and the local oscillator signals, DC offset and the need for a 30 V power supply, they do not solve the many other problems addressed by the present invention. In addition, they require a complicated multi-loop architecture to achieve desired phase-noise performance. This complicated multi-loop architecture is undesirable because of the added expense and difficulty in system-board design it creates.
Therefore, what is needed is a broadband tuner that does not require radiation shielding, generates less heat, is smaller, more robust, less expensive and easier to integrate into the system board.